`struct_field_attributes` disappear when running `rustfmt`

[rbtying]

rust-lang/rust#38814 added struct_field_attributes to Rust, however,

the file:

#![feature(struct_field_attributes)]

struct Foo {
    bar: u64,

    #[cfg(test)]
    qux: u64,
}

fn do_something() -> Foo {
    Foo {
        bar: 0,

        #[cfg(test)]
        qux: 1
    }
}

fn main() {
    do_something();
}

gets formatted to

#![feature(struct_field_attributes)]

struct Foo {
    bar: u64,

    #[cfg(test)]
    qux: u64,
}

fn do_something() -> Foo {
    Foo { bar: 0, qux: 1 }
}

fn main() {
    do_something();
}

which does not compile (it's lost the #[cfg(test)] in do_something)